1. Field of the Invention
This invention relates to a material testing machine which drives a hydraulic cylinder to apply load to specimens.
2. Description of the Related Art
Generally, a material testing machine applies various loads to test pieces such as specimens by driving a loading mechanism. In the case of fatigue testing done on a specimen, for example, vibration is continuously applied to the specimen. And a hydraulic cylinder is used as the loading mechanism for applying vibration to the specimen.
In such a material testing machine that drives a hydraulic cylinder, a hydraulic pressure source including a hydraulic pump is used to supply hydraulic oil to the hydraulic cylinder. This hydraulic pressure source has a motor for driving the hydraulic pump, and a relief valve for maintaining constant a supply pressure of the hydraulic oil supplied the hydraulic cylinder during an execution of material testing.
In recent years, in order to attain power-saving, a material testing machine has been proposed which reduces power consumption by the hydraulic pressure source (see Japanese Unexamined Patent Publication No. 2011-185755). Since the power consumption by the hydraulic pressure source depends mainly on the rotational frequency of the motor for driving the hydraulic pump and the supply pressure of hydraulic oil, the material testing machine described in Japanese Unexamined Patent Publication No. 2011-185755 inhibits unnecessary power consumption by changing the rotational frequency of the motor by inverter control based on the strokes of a cylinder rod of the hydraulic cylinder.
The material testing machine described in Japanese Unexamined Patent Publication No. 2011-185755 determines, by calculation using coefficients derived from some characteristics of a hydraulic circuit, a required discharge rate from a discharge pump during a test execution or, in other words, a required flow rate of hydraulic oil flowing through a supply route from the discharge pump to the hydraulic cylinder (see Japanese Unexamined Patent Publication No. 2011-185755, paragraph [0043]). However, there can be a divergence between a required flow rate calculated and an actually required flow rate. In such a case, more electric power is consumed than is necessary.